Components of the Earth

Abstract

The anatomy of the planet Earth is the basic backbone of the chapter. The different natural processes (both constructive and destructive) that shape the morphology of the Earth have been explained with ground zero observations. The various agents that play roles in changing the configuration of the Earth is highlighted with special reference to super cyclone AILA that caused considerable alteration of Sundarban estuaries during 2009 in terms of hydrological parameters. A bird’s eye view of the different types of coral is a bonus to the readers of the chapter.

Appendices

Annexure 2A: Common Types of Corals

Sl. No.	Genus	Representative
1.	Pocillopora	Pocillopora damicornis
2.	Madracis	Madracis kirbyi
3.	Acropora	Acropora hemprichii
4.	Montipora	Montipora aequituberculata
5.	Astreopora	Astreopora myriophthalma
6.	Pavona	Pavona varians
7.	Pachyseris	Pachyseris rugosa
8.	Siderastrea	Siderastrea savignyana
9.	Pseudosiderastrea	Pseudosiderastrea tayami
10.	Coscinaraea	Coscinaraea monile
11.	Psammocora	Psammocora contigua
12.	Cycloseris	Cycloseris cyclolites
13.	Goniopora	Goniopora planulata
14.	Porites	Porites lichen
15.	Favia	Favia speciosa
16.	Favites	Favites halicora
17.	Goniastrea	Goniastrea pectinata
18.	Platygyra	Platygyra sinensis
19.	Leptoria	Leptoria phrygia
20.	Hydnophora	Hydnophora microconos
21.	Leptastrea	Leptastrea transversa
22.	Cyphastrea	Cyphastrea microphthalma
23.	Echinopora	Echinopora lamellosa
24.	Plesiastrea	Plesiastrea versipora
25.	Galaxea	Galaxea fascicularis
26.	Merulina	Merulina ampliata
27.	Acanthastrea	Acanthastrea echinata
28.	Lobophyllia	Lobophyllia corymbosa
29.	Symphyllia	Symphyllia radians
30.	Mycedium	Mycedium elephantotus
31.	Polycyathus	Polycyathus verrilli
32.	Heterocyathus	Heterocyathus aequicostatus
33.	Heteropsammia	Heteropsammia sp.
34.	Tubastrea	Tubastrea aurea
35.	Dendrophyllia	Dendrophyllia coarctata
36.	Turbinaria	Turbinaria peltata

Annexure 2B: Changing Hydrological Face of Indian Sundarban Estuary due to Super Cyclone AILA

2.2.1 Introduction

In tropical and some subtropical areas, organized cloud clusters form in response to perturbations in the atmosphere. If a cloud cluster forms in an area sufficiently away from the Equator, then Coriolis accelerations are not negligible and an organized, closed circulation may occur. A tropical system with a developed circulation, but with wind speeds of less than 17.4 metres/second (i.e. 63 km/h or 39 mph), is termed a tropical depression. Given that conditions are favourable for continued development (basically warm surface waters, little or no wind shear and a high pressure area aloft), this circulation can intensify to the point where sustained wind speeds exceed 17.4 metres/second, at which time it is termed a tropical storm. If development continues to the point where the maximum sustained wind speed equals or exceeds 33.5 metres/second (121 km/h or 75 mph), the storm is termed a cyclone (Indian Ocean and Bay of Bengal), typhoon (Western Pacific) or hurricane (Atlantic and Eastern Pacific).

The India Meteorological Department (IMD) classified cyclone on the basis of sustained wind speed in to six major types (Table 2A.1). Considering the speed of AILA (110 km/h) on May 25, 2009, in the Gangetic delta stretch, it can be designated as severe cyclonic storm (SCS). AILA was formed in the central Bay of Bengal as the net output of several factors. Around May 20, 2009, monsoon initiated at Andaman. Under its influence, the southerly surge over the region increased. It resulted in increase in the horizontal pressure gradient and the north–south wind gradient over the region. Hence the lower level horizontal convergence and relative vorticity increased gradually over the southeast Bay of Bengal. This condition triggered the development of the upper air cyclonic circulation extending up to mid-tropospheric level on May 21 over the southeast Bay of Bengal and associated convective cloud clusters persisted over the region. Under the influence of the cyclonic circulation, a low pressure area developed over the southeast Bay of Bengal on 22nd May morning. It laid over east central and adjoining west central Bay of Bengal on 22nd evening. It concentrated into a depression and lay centred at 1130 h IST of 23rd near Lat. 16.5° N/Long 88.0° E about 600 km south of Sagar Island, the largest island in Indian Sundarbans out of 102 island clusters.

Table 2A.1 Tropical cyclone classifications (all winds are 10-min averages)

According to INSAT imageries, a low-level circulation developed over South Bay of Bengal on May 21, 2009, at 0830 h IST. It developed into a Vortex with centre 11.5° N/ 85.5° E and intensity T1.0 at 1730 h. IST on the same day. It gained intensity of T1.5 corresponding to depression with centre 16.5° N/88.0° E at 1130 h IST of 23rd May. It was the shear pattern at the time of cyclogenesis with maximum convection lying to the southwest of the system centre.

Considering the environmental factors for cyclogenesis, the wind shear between the layers (150–300) hPa and (700–925) hPa was 05–10 knots on 21st and 22nd May according to METEOSAT observations. It became 10–20 knots on 23rd May. However, it continued to be 05–10 knots in the northeast sector of the system. The sea surface temperature (SST) was warmer (about 28 °C) over central and north Bay of Bengal, being 0.5 to 10 °C above normal. There was maximum lower level convergence to the southeast of the system centre. Similarly, the upper level divergence and the lower level relative vorticity were higher around the system centre. The system could gain upper level divergence as the upper tropospheric ridge roughly ran along 17° in association with an anti-cyclonic circulation located near lat. 17° N and long. 94° E. The quickscat-derived wind speed was about 10–15 knots on 21st and 22nd. It became 15–20 knots on 23rd. However, the windsat observations indicated 25–30 knots wind on 23rd in association with the system. The wind speed was relatively stronger in the southeast sector due to strong southerly surge of the monsoon current. All these observations indicate that the environmental factors were favourable for intensification of the system.

The cyclone retained its intensity for about 15 h after it hit the landmasses as it was close to the Bay of Bengal. It laid centred over the Gangetic delta for a considerable period of time, ascertaining the availability of moisture. Due to occurrence of AILA, there was intrusion of saline water from Bay of Bengal into the Hooghly–Matla estuarine system. The present study was conducted on May 18, 2009 (before the AILA event, and even before the initiation of monsoon at Andaman), and May 29, 2009, just 4 days after AILA in 12 different stations in and around the Matla River. The study was extended further 10 days after the incidence to observe the behaviour of the selected hydrological parameters.

2.2.2 Materials and Methods

The entire network of the investigation encompassed the monitoring of hydrological parameters on May 18, 2009 (pre-AILA phase, designated as Phase A), May 29 (post-AILA period, referred to as Phase B) and June 4 (extended post-AILA period, referred to as Phase C) in 12 sampling stations in and around the Matla estuarine system of Indian Sundarbans (Table 2A.2). A GPS was used to fix the coordinates of sampling sites.

Table 2A.2 Location of the sampling stations

The surface water salinity was recorded by means of an optical refractometer (Atago, Japan) in the field and cross-checked in laboratory by employing Mohr–Knudsen method (Strickland and Parsons 1972). The correction factor was found out by titrating the silver nitrate solution against standard seawater (IAPO standard seawater service Charlottenlund, Slot Denmark, chlorinity = 19.376‰). Our method was applied to estimate the salinity of standard seawater procured from NIO and a standard deviation of 0.02% was obtained for salinity. The average accuracy for salinity (in connection to our triplicate sampling) is ±0.28 psu. Glass bottles of 125 ml were filled to overflow from collected water samples and Winkler titration was performed for the determination of dissolved oxygen. The pH was recorded with a portable pH metre (Hanna, USA), which has an accuracy of ±0.1.

2.2.3 Results and Discussion

Tropical cyclones and storms are more common in the Bay of Bengal. They severely affect the eastern coast of India as compared to that of the Arabian Sea. According to Koteswaram (1984), there were about 346 cyclones that include 133 severe ones in the Bay of Bengal, whereas the Arabian Sea had only 98 cyclones including 55 severe ones between the years l891 and l970. These cyclones with tremendous speed hit the coastline and inundate the shores with strong tidal wave, severely destroying and disturbing coastal resources. The intrusion of seawater in to the upstream riverine zone through estuaries, creeks and inlets has high probability to alter the chemical composition of the aquatic phase, which is a subject of present discussion.

The present study recorded the increase of surface water salinity by 15.65%, 19.13%, 18.70%, 18.78%, 18.34%, 16.56%, 24.18%, 22.22%, 21.79%, 14.32%, 20.45% and 23.80% at Canning (Stn. 1), Sandeshkhali (Stn. 2), Sonakhali (Stn. 3), Gosaba (Stn. 4), Amlamethi (Stn. 5), Sajnekhali (Stn. 6), Kultali (Stn. 7), Chotomollakhali (Stn. 8), Satjelia (Stn. 9), Kumirmari (Stn. 10), Pakhiralaya (Stn. 11) and Netidhopani (Stn. 12), respectively, and this increase is significant at 1% level (Tables 2A.3 and 2A.4). Salinity of water can greatly affect organisms surviving in the system. In the Chattonella marina, it has been stated that the salinity of the water can contribute to the growth rate of harmful algal blooms in that area. These blooms may pose an adverse effect on secondary production especially by killing local fish (Liu et al. 2007). While salinity has been known to have a positive impact on the growth of algal blooms, high salinity has been known to stunt algae growth, which can affect the productivity of the system.

Table 2A.3 Variations of hydrological parameters during different phases before and after AILA

Table 2A.4 ANOVA for surface water salinity

The pH value also increased in some stations (e.g. 0.12% increase at Sonakhali, Gosaba, Sajnekhali, Kumirmari and Netidhopani) as confirmed by the ANOVA (Table 2A.5). High pH can affect the benthic community by way of precipitating the heavy metals from the aquatic phase to underlying sediment compartment (Mitra and Choudhury 1992, 1993a, b).

Table 2A.5 ANOVA for surface water pH

The DO level did not vary significantly between the stations (F_obs = 1.682 < F_crit = 2.818) in the study area. However, after the AILA the DO level in the surface water of the selected stations decreased within the range 2.74% to 29.91% (exception: 2.44% increase at Sajnekhali), and this decrease was significant at 1% level as revealed from ANOVA (Table 2A.6). The low DO observed during the AILA period (Phase B) may be attributed to high salinity, which may result in the mortality of aquatic life or affect their metabolic process and rate. Depletion in dissolved oxygen (and resulting decrease in water quality) can cause major shifts in the kinds of aquatic organisms found in water bodies, and evidences of such shifts, if any, need to be properly documented for such situation originating from tidal surges and intrusion of seawater.

Table 2A.6 ANOVA for dissolved oxygen (DO)

We conducted a similar study in the same locations 10 days after the AILA to evaluate the status of hydrological parameters and observed a recovery trend in all cases. A decrease in aquatic salinity is observed (7.14% at Canning, 7.45% at Sandeshkhali, 10.13% at Sonakhali, 14.26% at Gosaba, 10.74% at Amlamethi, 13.86% at Sajnekhali, 14.77% at Kultali, 14.09% at Chotomollakhali, 8.96% at Satjelia, 12.09% at Kumirmari, 12.76% at Pakhiralaya and 12.52% at Netidhopani) and the decrease is significant as confirmed through ANOVA (Table 2A.7). This is a clear indication of restoration of the water quality (in terms of salinity) after the AILA incidence. The relatively high salinity in the extended period (phase C) in comparison to pre-AILA condition (Phase A) is a normal trend of the geographical locale as pointed out earlier by several workers (Mitra et al. 2009). Such increase will continue till the end of June, and with the onset of monsoon the salinity value will drop. However the decreased percentage of Phase C in relation to post-AILA condition (Phase B) is a clear indication of the gradual restoration of the system, which could have been more effective if the dilution of the system, could be increased through fresh water discharge. This is, however, not feasible in this geographical locale as huge siltation of Bidyadhari basin has cut-off the fresh water supply of Bhagirathi-Ganga to this sector. The restoration of pH and DO through increased level was also confirmed through ANOVA (Tables 2A.8 and 2A.9).

Table 2A.7 ANOVA for salinity of post-AILA (Phase B) and extended post-AILA (Phase C)

Table 2A.8 ANOVA for pH of post-AILA (Phase B) and extended post-AILA (Phase C)

Table 2A.9 ANOVA for DO of post-AILA (Phase B) and extended post-AILA (Phase C)

The effect of AILA was severe resulting in the death of people, damages to properties and alterations in the physico-chemical characteristics of the soil and water. Embankments were destroyed, agricultural fields and freshwater ponds lost their productivity due to intrusion of saline water and many people became homeless. 70 island dwellers died in the Indian Sundarbans region. Over 8000 people could not be traced and about a million became homeless in India and Bangladesh. The embankments, which act as the line of defence of the villages, were completely smashed by tidal surges. Out of 3500 km long stretch of embankment, 400 km were completely broken and another 565 became highly vulnerable. The agricultural fields were submerged by sea water, and all standing crops were destroyed. The fresh water ponds were transformed into brackish water bodies, and several fish species of saline water were recovered from these ponds. The entire spectrum of damages is summarized in Table 2A.10.

Table 2A.10 Damage spectrum of AILA

The geographical and geological setting of Indian Sundarbans has made the area extremely vulnerable to cyclones and tidal surges. Dube et al. (2004) created models to simulate the storm surges from past cyclones in the head of the Bay of Bengal (Orissa, West Bengal and Bangladesh) and discussed the formation and effect of each of these storms that were modelled. The extent of storm surges in the Bay of Bengal region and the causes behind extreme sea levels were also analysed by several workers (Dube et al. 1997). The main reasons behind the vulnerability are:

• Coastal waters (shallow bathymetry extending tens of kilometres offshore)

• Convergence of the bay

• High astronomical tides

• Thickly populated low-lying islands

• Favourable cyclone tracks impacting perpendicular to coastline

• Innumerable inlets and river systems

The situation of deltaic Sundarbans coincides with most of these points. Being located at the apex of Bay of Bengal, the islands of the deltaic complex (102 in numbers) are criss-crossed with networks of creeks and inlets. The geological formation of Sundarbans is of comparatively recent origin. Till a few thousand years back, the whole tract was under the sea. The deposition of debris and formation of Sundarbans delta occurred recently with the change of main course of River Ganga from the Bhagirathi to Padma towards the east between the twelfth and fifteenth century A.D. Basically this was the result of Bengal basin suffering from neotectonic movement and an easterly tilt. During the sixteenth century, the flow of Ganga shifted almost totally eastwards into River Padma (now in Bangladesh) and the Matla/Bidyadhari rivers which had formed innumerable network of creeks in the delta got completely cut-off from the sweet water sources. This is another major reason behind increased vulnerability in and around Matla estuary to cyclone induced waves as the pressure of fresh water supply cannot prevent the saline intrusion to upstream zone. The Gangetic delta of Indian sub-continent is a cyclone prone zone, and the event of AILA is an eye opener to think of a strong action plan, so that the damage can be minimized in future. In this context, it is extremely essential to dredge the Bidyadhari channel so that the fresh water from western sector (Bhagirathi-Ganga-Hooghly channel) sourced from Farakka barrage may be diverted in the central Indian Sundarbans in and around the Matla estuary. This will not only reduce the salinity intrusion during extreme tidal surges and tropical storms but may also lead to development of a congenial and uniform salinity profile in the region.